Squeeze the 2 parts to release the mechanism, window pops out a bit then returns a little, as you say. If you push (gently) the little window itself you'll see how the mechanism moves outwards - a final, slight outward press from your thumb on the outer part will complete the movement and you'll see the inner part (labelled "close") rise then fall to secure the window in the open position. To release it, lift this "close" part from its locked position and the window will flop back towards the van. Press the outer part to secure it shut, the mechanism held by the 2 parts you squeeze to release.
You've probably worked it out before you reached the end of this explanation!
